The 'other' line was delivered portentously because it was intended to be the set up for the final trilogy.

In the final play when the sister and Leia were merged into one character Leia doesn't actually play a major role in the finale, 'other' than being the trigger for Luke snapping and going right to the edge of the cliff before coming back again.

The only character that carries that degree of potency in that duel to retroactively justify the manner of that line is Vader himself.

He saves Luke, he kills the Emperor and he saves his true self.